In a power semiconductor device, a substrate upon which a power semiconductor chip is formed is bonded to abase plate. The base plate is screwed or otherwise attached to a heat sink, in order to dissipate the heat generated by the electrification in the power semiconductor chip.
In this type of semiconductor device, in order to improve the contact of the base plate and the heat sink with one another, the base plate is warped into a convex shape on the heat sink side thereof. On the periphery of the base plate, via holes are provided in order to insert screws therethrough to secure the base plate to a heat sink.
The periphery of the base plate is spaced from the heat sink upon initial placement of the base plate on the heat sink. When tightening the base plate with a screw, the periphery of the base plate will be pushed down and will become distorted into a shape matching the profile of the heat sink.
If the distortion resulting from the screwing operation propagates into the interior of the base plate, where the semiconductor device is bonded thereto, the power semiconductor chip will be affected by forces which tend to peel it or shear it from the base plate. As a result, bonding parts are destroyed, or internal stresses in the chip will increase, which degrades in the long-term reliability of the power semiconductor device.